A conventional fan generally comprises a current-limitation circuit used to protect circuit from over current. When a fan current of the fan increases and reaches to a predetermined level, the current-limitation circuit limits the fan current and enables the fan current to stop rising. Therefore, when the fan is locked and incapable of operating normally, maximum consumption power can be limited within an allowable range to avoid long-term over current to lead destruction of electronic devices. However, when the fan encounters an external force (such as locked fan blade or dust accumulation) and is not completely locked therefore producing higher current level, in the mean time, the current-limitation circuit is not activated yet so that electronic devices might be destructed by higher current level.
To improve mentioned disadvantage, with reference to R.O.C. patent publication No. 540902, this invention utilizes a two-stage current limiter to prevent a high pulse current from passing through coil windings under abnormal operation. However, the fan current is a ripple current and peak value of the ripple current is 1.5 times the average value. If trigger point of the current-limitation circuit is lower than peak value of the ripple current, the normal operating fan current probably increases to peak value of the ripple current instantaneously thereby activating the current-limitation circuit in error. Consequently, designer generally designs trigger point of the current-limitation circuit higher than 1.5 times the average value of the ripple current. But, before activation of the current-limitation circuit, the fan still sustains long-term over current to lead destruction of electronic devices.